1. Field of the Invention
The present invention relates to a magnetoresistive effect element (MR element) and particularly relates to a configuration of a spacer layer.
2. Description of the Related Art
Reproducing heads having high sensitivity and high output are in demand in conjunction with condensing of high recording density in hard disk drives (HDD). A spin valve head has been developed as an example of this type of reproducing head. A spin valve head includes a nonmagnetic metal layer and a pair of ferromagnetic layers positioned to contact both sides of the nonmagnetic metal layer. The magnetization direction of one side of the ferromagnetic layers is pinned in one direction (hereinafter, this type of layer is referred to as a magnetization pinned layer), and the magnetization direction of the other side of the ferromagnetic layers freely rotates in response to an external magnetic field (hereinafter, this type of layer is referred to as a magnetization free layer). When an external magnetic field is applied, the relative angle of the spins between the magnetization pinned layer and the magnetization free layer change so that a magnetoresistive change is realized. Typically, the magnetization direction of the magnetization pinned layer is pinned by utilizing the exchange coupling force of an anti-ferromagnetic layer.
Realizing further condensing of high recording density requires a read gap (a space between upper and lower shield layers) to be reduced. However, when reducing the read gap to about 20 nm, placing an anti-ferromagnetic layer within the read gap becomes difficult. Therefore, a configuration has been developed in which a pair of magnetization free layers is arranged on both sides of a spacer layer. According to this configuration, reduction of the read gap is easily realized because an anti-ferromagnetic layer is unnecessary.
With either configuration, the spacer layer is a necessary component to realize a magnetoresistive change, and promising spacer layer materials have been developed to achieve large magnetoresistive ratio (hereinafter referred to as MR ratio). Oxide semiconductors such as ZnO and TiO are known examples.
For example, U.S. Patent Publication No. 2008/0062557 discloses the technology in which a nonmagnetic metal layer is disposed between an oxide semiconductor layer such as ZnO or TiO, etc., and a ferromagnetic layer composed of CoFe or the like. Copper, gold, silver, and the like are given as examples of nonmagnetic metal layers.
The ferromagnetic layer adjacent to the spacer layer is generally composed of Co, Ni, Fe, or the like as a primary component, and when these elements are arranged to contact the oxide semiconductor layer, such an arrangement leads problems that the ferromagnetic layer is oxidized by the oxidative effect of oxygen contained in the oxide semiconductor layer, that the oxidation reduces the polarizability and thereby reduces the MR ratio. In the technology described in US2008/0062557, the nonmagnetic metal layer such as copper, gold, silver, or the like can be expected to prevent oxidation of the ferromagnetic layer. However, because a simple metal such as copper, gold, silver, or the like is more likely to become island-shaped when the film thickness is thin, a certain film thickness is required in order to function as an oxidation preventing film. On the other hand, when the film thickness is too thick, electrons are easily scattered which reduces the MR ratio.
An object of the present invention is to provide a magnetoresistive effect element that can prevent oxidation of a magnetic layer adjacent to a spacer layer and realize a large MR ratio.